Well, openERP or openbravo were what I would have recommended ten years ago, but due to their commercialization aren’t really relevant any longer. If I personally was setting this up for myself I would probably use redmine and a plugin that gives redmine the invoice functionality. However I wouldn’t call it simple for a first timer to pull off, but if redmine is mastered you will find very extensible and customizable to any particular project’s needs.

I use pancake, works pretty well. It’s paid, but only a one-time payment and you get the code.

EDIT: here’s the link: https://www.pancakeapp.com/